The present invention relates to magnetic bearings and, more particularly, to precise positioning of the air gaps between relatively movable parts in such bearings.
Magnetic bearings are a preferred bearing used where frictionless, longlife support is needed. The absence of friction in a magnetic bearing allows significant improvement in performance of any precision servo system, such as used in space sensor gimbal suspensions and reaction wheels for spacecraft attitude control, where the combination of better control system performance plus long life are much sought after parameters. Other uses include long life spindle bearings and machine shop applications.
Three commercially available proximity sensors include the Schaevtz variable inductance sensor, the Kaman eddy current sensor, and the Bently Nevada eddy current sensor. All these devices are temperature sensitive, and the eddy current type is particularly so. All depend on use in matched pairs to measure displacement differentially, thus to achieve modest temperature insensitivity over a limited range. The long term null stability of all these devices depends on other reactive circuit elements in an alternating current bridge circuit.
In a magnetic bearing application, a proximity sensor is used to control the radial clearance between the fixed and moving member via a servo system. If the proximity sensor null drifts, for example as a function of temperature, the bearing ceases to function as a friction free device and may even destroy itself due to contact between the stationary and moving members.